Color is important for segmenting objects from backgrounds, which can in turn facilitate visual search in complex scenes. However, brain areas that control overt visual orienting (i.e., saccadic eye movements) are not believed to have access to color (Schiller et al., 1979), despite massive visual corticotectal projections (Lock et al. 2003), which include areas traditionally associated with color processing (e.g., V4). Here, we show the first evidence that neurons from the intermediate layers of the monkey superior colliculus (SC), a critical structure for both overt and covert visual orienting (Fecteau & Munoz, 2006; Ignashchenkova et al., 2004), can respond to pure chromatic stimuli with the same magnitude as a maximum contrast luminance stimulus. In contrast, neurons from the superficial SC layers showed little color response. Crucially, visual onset latencies were approximately 30ms longer for color, implying that luminance and chrominance information reach the SC through distinct pathways, and that the color response cannot be due to residual luminance signals. Furthermore, these differences in visual latency translated directly into differences in saccadic reaction time (SRT) between color and luminance, which closely match SRT differences reported in humans (White et al., 2006). These results demonstrate that the saccadic eye movement system can signal the presence of pure chromatic stimuli only one stage from the brainstem premotor circuitry that drives the eyes.

The authors thank Ann Lablans, Becky Cranham, Donald Brien, Sean Hickman and Mike Lewis and for technical assistance. This project was funded by the Human Frontiers Science Program, Grant RGP0039-2005-C, and the Canadian Institutes of Health Research. DPM was supported by Canada Research Chair Program.